1. Field of the Invention
This invention relates to compositions useful as adhesives and more particularly to the preparation of epoxy-based adhesive compositions having a desirable combination of rheological properties.
2. Brief Discussion of the Related Art
Epoxy resins have traditionally been extensively used as components of adhesives, especially structural adhesives, in view of their desirable combination of properties. Epoxy resin-based adhesives commonly exhibit excellent strength, toughness, corrosion, moisture and solvent resistance, high load-bearing properties, good adhesion and electrical properties, good dimensional stability, hardness and low shrinkage upon curing. Typically, epoxy resins may be formulated as one-component adhesives, together with other components such as fillers, tougheners, curatives and the like, which are applied in paste form between two substrate surfaces and then heated to effect curing (crosslinking) of the epoxy resin.
Good rheological control is important for such epoxy paste adhesives, as it will be desirable for the adhesive to thin under shear to allow for easy handling and application to a substrate surface. However, once it has been applied, the bead, ribbon or layer of adhesive must not slump or sag to an appreciable extent or be easily moved on or separated from the substrate surface by externally applied forces such as gravity or the impingement of liquids such may be encountered when the substrate surface bearing the adhesive is exposing to a washing or dipping operation (as typically occurs, for example, during assembly of a vehicle). Such movement or separation of the applied adhesive creates a number of problems, including degraded adhesive bond quality and contamination of the aqueous liquid (e.g., phosphating bath) with the adhesive. While a number of different types of rheological control agents have been proposed for use in epoxy paste adhesives in order to control their viscosity and yield value properties, further improvements in such systems are still highly desired by the end-users of these adhesives.